1. Field of the Invention
The present invention relates to an image reader for separating light from an image of an original into a plurality of color light components to read the same by a plurality of linear image sensors, and more particularly, it relates to a system which precludes misalignment between respective elements of the reader so that there is no reduction in picture quality.
2. Description of the Prior Art
In a process color scanner or the like, an image of an original must be read sequentially along scanning lines for each color component of light from the original image. Thus, such system, employ an image reader to separate light from the original into three color components, i.e., red, blue and green, and to photoelectrically convert the same into electrical signals by respective linear image sensors.
FIG. 1 illustrates a conventional image reader 2. The image reader 2 has an image-formation lens system 3. Light L from a read line RD enters the image reader 2 through the image-formation lens system 3.
The light L is separated in three directions by first and second half mirrors 4a and 4b to provide three image-formation beams l.sub.R, l.sub.B and l.sub.G. Filters 5R, 5B and 5G for extracting the red, blue and green components are provided along the paths of the image-formation beams l.sub.R, l.sub.B and l.sub.G respectively. Linear image sensors 6R, 6B and 6G are provided behind the filters 5R, 5B and 5G, respectively. In order to read intensity levels of red, blue, and green components respectively supplied through the filters 5R, 5B and 5G, each of the linear image sensors 6R, 6B and 6G is provided with a one-dimensional array of light receiving cells 7.
Referring to FIG. 1, the image sensor 2 and the original 1 are moved with respect to each other along a subscanning direction Y at a velocity V. Light from a strip-shaped region S is received by the respective linear image sensors 6R, 6B and 6G during a prescribed time .DELTA.t. Thus, the region S defines a single scanning line having a width of (V.multidot..DELTA.t). The entire image of the original 1 is sequentially scanned and read by sustaining the relative movement between the original 1 and the image reader 2 while repeating image reading for each scanning line.
When misalignment occurs between the linear image sensors 6R, 6B and 6G and the half mirrors 4a and 4b in the image reader 2, lines to be read by the linear image sensors 6R, 6B and 6G are displaced. FIG. 2 illustrate such a state, in which the respective linear image sensors 6R, 6B and 6G take in image data from different read lines RR, RB and RG, although they should read image data from the same line. As a result, degradation in picture quality (such as color deviation) is caused when an image is reproduced by combining data read by the misaligned image sensors 6R, 6B and 6G.
Such misalignment is not only caused when the apparatus is assembled, but can also be caused after assembly by thermal expansion of respective members or mechanical deformation in transportation, etc. Thus, it is difficult to prevent picture quality degradation. Indeed, picture quality degradation caused by misalignment can be caused even if the elements constituting the image sensor are carefully assembled and adjusted. Furthermore, since it is the read position of the original 1 itself which is deviated by misalignment, a quality image cannot be reproduced even if signal processing techniques such as color masking are employed.
The problem is typically encountered when highly accurate image reading is required such as during process color scanning. However, the problem also exists in other apparatuses for reading color images, such as a color facsimile and a color copying apparatus.